For detection of protein from a sample using an immunological measurement method, it is known that an immune reaction between protein and antibodies or antigens is related to the tertiary structures of antibodies or antigens. It is also known that the protein detection accuracy decreases if the tertiary structures of antibodies or antigens vary. The in-vitro diagnostic used in this type of measurement usually is configured to immobilize a blocking agent such as bovine serum albumin (BSA) or polyethylene glycol (PEG) to a support to stabilize the tertiary structure of an antibody or an antigen bound with the support.
In addition, for detection of a particular signal in the measurement using the immunological measurement method, it is known that nonspecific coupling between protein in a sample and an antibody or an antigen, or a nonspecific signal caused by nonspecific adsorption of protein in a sample to a support adversely affects the detection of the particular signal. Accordingly, a blocking agent, etc. is added to a measurement solution in which a sample and a reagent component are mixed, in order to suppress occurrence of a nonspecific signal. In this case, the blocking agent may be a BSA, hydrolyzed vegetable protein, immune globulin, or casein. The blocking agent may be the same as or different from the aforementioned blocking agent used to stabilize the tertiary structure.
Even if a blocking agent to stabilize the tertiary structure of an antibody or an antigen is immobilized to a support of an in-vitro diagnostic, nonspecific binding of protein in a sample and an antibody or an antigen, or nonspecific adsorption of protein in a sample to a support cannot completely be prevented if a blocking agent is not added to the measurement solution. Thus, occurrence of a nonspecific signal cannot be sufficiently suppressed.
On the other hand, if a blocking agent is added to the measurement solution, the blocking agent has to be stirred throughout the measurement solution. Accordingly, even if there is dead volume, it is necessary to add a large amount of blocking agent in accordance with the volume of solution to be used, not the immune reaction amount. A blocking agent is generally mixed to a buffer solution prior to addition of the blocking agent to the measurement solution. However, since the blocking agent usually is protein or hydrolyzed protein, the blocking agent is difficult to be preserved stably in a liquid state. In addition, the measurement solution in which such a blocking agent is added has limitations for preserving conditions, for example, the solution can be preserved only for a short time at room temperature, or the solution needs to be preserved in a refrigerator. Furthermore, if the measurement solution into which such blocking agent is added is preserved in a refrigerator, the solution has to return to room temperature before measurement is carried out.